The entire surface of the human body is covered by a layer of skin, which is considered to be the largest organ in the body. It serves as a barrier between the internal organism and the external environment, to prevent toxic materials from entering into the body and to retard excessive body water loss. In addition, it also plays a major role in temperature regulation, vitamin synthesis, excretion, sensory perception, and processing of antigenic substances.
The skin consists of three major layers of tissue. From inside out, the layers are the subcutaneous tissue, the dermis, and the epidermis.
The epidermis is the most superficial layer of the skin. It is divided into a living inner layer of viable cells (stratum Malpighii) and an outermost laminated sheet of dry anucleate flattened horny cells (stratum corneum or horny layer).
The lowermost cell layer of the epidermis (stratum basale or stratum germinativum) consists of the basal cells. Basal cells are continually moving up to the surface of the skin and undergo modification in a process called keratinization, and are eventually shed. The normal cell turnover time from the stratum basale to the skin surface and shedding is approximately twenty-eight days. The stratum spinosum lies immediately over the basal layer. This stratum consists of several layers of cells, and the shape of these spinous cells becomes progressively more flattened in a plane parallel to the surface of the skin as they move outward. Above the spinous cells is the stratum granulosum, which consists of one to three layers of cells. The granular layer is most highly developed in the regions where abundant keratin is produced. Keratins are fibrous and insoluble proteins which are largely responsible for the toughness of the protective outer covering of the skin. The next stratum is the stratum lucidum, which consists of cells that are on the way to becoming the flat, anucleate and dead cells that constitute the stratum comeum. The stratum corneum is formed and continuously replenished by the slow upward migration of cells from the germinative basal layer of the epidermis. The entire stratum corneum is replaced about every two weeks in mature adults.
The condition of dry and chapped skin, which afflicts everyone at some time, is visually characterized as a slight roughening and less flexibility in the feel of the skin surface. Among dermatologists, this condition is called xerosis, in which the skin loses its suppleness, forming cracks and fissures. Environmental factors play an important role in bringing about this condition. Decreased humidity contributes to water loss from the skin surface, dry and cold winds increase evaporation by convection, and low temperatures decrease stratum corneum extensibility. The increased use of synthetic detergents also helps to dehydrate the stratum corneum.
The physical appearance of the slin is solely governed by the state of the stratum corneum. It has been demonstrated that the prime factor responsible for dry skin is the lowered moisture content of the stratum corneum. The factors that influence the state of hydration of the stratum corneum can be classified into three general categories: the rate at which water reaches the stratum comeurn from layers beneath it; the rate at which water leaves the skin surface by evaporation; and the ability of the stratum corneum to hold moisture.
The stratum corneum receives water from the sweat glands and from the underlying tissues by diffusion. At the same time, it loses water to the environment by evaporation. Under normal conditions, the rate at which water diffuses from the underlying tissues to the skin surface is slow and uniform. Experiments indicate that the major barrier against water loss over most areas of the body is a very thin barrier at the base of the stratum comeum, which separates the stratum comeum from the easily available water of the underlying tissues and makes it dependent upon the surrounding environment for the moisture. As a result, at low relative humidity, when water tends to be lost from the surface at a more rapid rate, the stratum corneum will tend to dry out.
The softness and flexibility of the skin is determined by the moisture content of the stratum corneum. Contrary to older beliefs, the amount of oil in the stratum corneum is not the essential factor in controlling the physical appearance of the skin. Thus pieces of hardened stratum corneum immersed in various oils do not regain their flexibility, whereas immersion in water increases their flexibility. However, the removal of the surface lipids of the skin after organic solvent treatment also brings about the feeling of dryness. This phenomenon demonstrates the water-holding ability of the skin lipids.
Various kinds of lipids are located in the intercellular region of the stratum corneum, which are called the intercellular lipids or the stratum corneum lipids. Stratum comeum lipids are composed mainly of ceramides, free fatty acids, and cholesterol, with small proportions of triglycerides, sterol esters, and cholesterol sulfate. The sphingolipid content is reported to reveal a direct relationship with permeability to water, while the neutral lipids are also suggested to make a definite contribution to the water-retention properties of the stratum corneum. Lipid compositions of different cell populations in pig epidermis are disclosed by Goldsmith, ed., Biochemistry and Physiology of the Skin, Oxford University Press, New York and Oxford, 1983, 364.
Dermal components of humans and animals have received much attention in the hope of identifying markers of biologic aging. The dermis is composed mainly of highly stable fibers, predominantly collagen and about 5% elastin fibers. Collagen has high tensile strength and prevents the skin from being torn by overstretching. Elastin is an elastic protein that maintains normal skin tension. It is the collagen-elastin fiber network that gives the skin its strength and elasticity. Hall (1976) The Aging of Connective Tissue, Academic Press, New York used "the rods and elastic band" model to demonstrate the network of the collagen bundles in human skin. The collagen bundles are loosely arranged in a rhomboid network with individual bundles lying at angles to one another. Intertwined amongst the collagen bundles lie single elastin fibers. The network of collagen bundles can be distorted by the application of a force in one direction, but it returns to its original form when the force is removed, in exactly the same fashion that a network of rigid rods will resume its shape if each crossing point is restricted by an elastic band.
Both the collagen bundles and the elastin fibers seem to undergo characteristic changes with time. Imayama and Braverman (1989) Am. J. Pathol. 134:1019 reported that there is a dynamic rearrangement of the collagen and elastic fibers during the growth period of rats. The collagen bundles uncoil, thicken and develop a lattice pattern of relatively straight bundles with age. As the collagen bundles straighten, however, they bend and dislocate the elastin fibers. During adulthood, elastin fibers become increasingly tortuous and impart a frayed or porous appearance to the skin surface. The elastin fibers become more stretched and therefore a decrease in their original elasticity results. These phenomena lead to the looseness, sagging and wrinkling of aged skin.
Skin care products can be used to prevent excessive water loss or to restore the high moisture content of the stratum corneum. There are two groups of cosmetic products available for the treatment of dry skin conditions, emollients and moisturizers.
Emollients, often termed skin conditioners, increase and maintain hydration by lubricating or occluding the skin surface. They reduce the evaporative loss of water from the outside of the skin and cause a buildup of water in the stratum comeum. Emollients include a very wide range of compounds. They are all water-insoluble materials. Petrolatum is the most efficient emollient for protecting dry skin. Lanolin (a fatty secretion from sheep's wool, which consists of a mixture of fatty acid esters of the sterols, lanosterol and agnosterol), fatty acids, fatty alcohols, triglyceride esters, wax esters, and esters of polyhydric alcohols are all common emollients. Idson (1992) Cosm. & Toil. 107: 69.
Moisturizers are composed of hygroscopic substances. They often contain humectants, substances that attract moisture to the skin, such as urea, glycerin, propylene glycol, sorbitol, pyrrolidone carboxylic acid (PCA), or sodium lactate, to impart or restore moisture to the stratum comeum. Loden et al. (1994) "Product Testing--Testing of Moisturizers" in Bioengineering of the Skin: Water and the Stratum Corneum, Elsner et al., eds., CRC Press, Boca Raton, Fla., 275.
In order to evaluate moisturizer efficacy, and the irritation and barrier destruction potentials of soaps and solvents, the term "transepidermal water loss" (TEWL) was introduced. It is used to indicate the amount of water vapor passing through the stratum comeum by passive diffusion. In other words, TEWL is a true reflection of stratum corneum barrier function for water only in the absence of sweat gland activity. Rothman, "Insensible Water Loss" in Physiology and Biochemistry of the Skin, University of Chicago, 1954, 233.
The mathematical principle governing the diffusion of water through stratum comeum is Fick's law. TEWL is calculated according to the following integrated form: EQU J.sub.s =K.sub.m D(c.sub.s /.delta.) eqn. 1
where J.sub.s =steady state flux of water (g cm.sup.-2 s.sup.-1);
K.sub.m =partition coefficient; PA1 D=diffusion coefficient of water (cm.sup.2 s.sup.-1); PA1 .delta.=thickness of the membrane (cm); PA1 c.sub.s =concentration gradient of water across the stratum comeum (g cm.sup.-3).
The water content of the innermost layer of the stratum comeum is in equilibrium with the adjacent moist granular layer, which is in turn in equilibrium with the drier environment surrounding the skin. Thus, there exists a concentration gradient of water within the stratum comeum that results in a continuous diffusion of water from within the body through the skin and into the environment. The wetter the surface layer, the smaller the concentration gradient and the smaller should be the TEWL.
Upon hydration, the stratum corneum thickness .delta. increases due to swelling of the tissue; the diffusivity D also increases with increasing water content. Thus, the net result of a change in stratum corneum hydration on TEWL is not always predictable. However, in healthy skin, D usually predominates and TEWL increases.
The introduction of the partition coefficient K.sub.m into equation 1 takes account of the fact that in the diffusion process the concentrations at the surfaces of the membrane are not necessarily equal to the concentrations in the external solutions. K.sub.m is defined as K.sub.m =c.sub.m /c.sub.s, where c.sub.m =concentration of H.sub.2 O in the membrane (g H.sub.2 O cm.sup.-3 of wet tissue); and c.sub.s =concentration of H.sub.2 O in the solution (g H.sub.2 O cm.sup.-3 of solution). Blank et al. (1984) J. Invest. Dermatol. 188.
It is an objective of the present invention to provide a compound that is capable of conferring a long-lasting skin care benefit. Currently available skin care products do not offer the convenient, long-term effects of the present invention. It is a further object of the present invention to provide a long-lasting compound that can be used in treating and preventing a dry skin condition, strengthening skin, or providing protection against UV light, for example by inducing a nucleophilic addition reaction to occur between the skin and the agent that provides these emolliency, moisturizing, strengthening or UV-protective effects. As a result, the agent is covalently bonded to skin proteins. Because new cells are continually being produced from the stratum basale to be shed, eventually, from the surface the binding period may last for possibly weeks to maintain the hydration and strengthening of the skin. Periodic application of the agent affords a virtually continuous maintenance of the beneficial effects, as the modified proteins work their way to the surface and reside in all the upper layers.